Line drivers including voltage-mode drivers are used in a number of applications such as mobile serializer-deserializer (SerDes) circuits, network switches and data center applications, and high speed PHY circuits. The main function of a line driver is to transmit a signal reliably across a medium (e.g., a line such as a conductor), in the presence of attenuation and distortion.
The required transmitted output swing specified by current standards (e.g., the IEEE standards) is technology independent. With technology nodes moving to smaller feature sizes, however, nominal supply voltage is also scaled down. For example, the 802.3ap standard for backplane Ethernet interface (e.g., IEEE KR) requires a minimum differential peak-to-peak voltage of 800 mV, which is greater than a nominal supply voltage that technologies below 28 nm can support. In a 16 nm technology, for instance, the supply voltage is approximately 800 mV. Some of the existing solutions increase the driver power supply and use a level shifter from the voltage-driver core to the power supply, which increases power consumption, or use two power supplies or a low drop-out (LDO) circuit. Other solutions use lower on-chip resistors, which sacrifices an important transmitter specification such as a return loss.